The present invention relates to an electric motor comprising a commutator and lever brushes which resiliently press thereagainst and are pivotable about a pivot axis, a connection line which extends in a direction perpendicular to the pivot axis between the center point of a brush contacting surface and the pivot axis being tangential to the outer surface of the collector.
Most of the time, electric motors of such a design are small sized motors used for the most different applications. A generic brush arrangement of an electric motor is described in German utility model 29605114. In this arrangement two opposite lever brushes press with their brush contacting portion against the commutator. The lever brushes are formed by a pressed shaped body consisting of graphite with conductive additives. A bearing eye which is put onto a pin provided on the brush cover is arranged opposite to the contact surface end of the lever brushes so that each lever brush is held to be pivotable about a pivot axis. An imaginary connection line between the contact point of the brush contacting surface and the pivot axis of the lever brush is tangential to the outer surface of the commutator. The reason is that such a brush arrangement is suited for both rotational directions. To be more specific, the lever brush should wear down in a uniform manner, independently of the rotational direction. A pigtail which communicates with corresponding terminal contacts is molded into the lever brush during the manufacturing or pressing process. The lever brushes are pressed onto the commutator by means of roll-type springs which are pressed onto the back of the lever brushes. Although such a construction has turned out to be useful, attempts are made to further reduce the size of the arrangement and to enhance miniaturization.
It is therefore the object of the present invention to improve an electric motor of the above-mentioned type, in particular the brush arrangement thereof, with respect to the space needed.
That object is achieved according to the invention in that each lever brush comprises a bearing bush rotatably arranged to be coaxial to the pivot axis, as well as a shaped body which is formed from a conductive material and firmly connected to the bearing bush and which forms the brush contacting surface and includes a connection point in direct electrical contact with the brush contacting surface for electrically connecting the lever brush, and that the bearing bush consists of a material more stable than that of the shaped body.
Thus, the invention offers the advantage that the rotational direction is still independent, that the connection point is in direct electrical communication with the brush contacting surface and that a bearing bush is used consisting of a more stable material. A more stable material means any material which is better or more durable as to its bearing characteristics than the material of the shaped body. This means that the wall thickness given to the bearing bush may be considerably smaller than the former wall thickness given to the pressed and shaped body in the area of the bearing eye. As a result, the pivot axis of the bearing point can be arranged closer to the commutator and the lever arm length can be reduced. Thanks to the use of a bearing bush consisting of a suitable stable material, wear created at the bearing point can also be reduced considerably. It is thereby possible to design the section of the shaped body which forms the brush contacting surface in such a manner that a wear length that is as great as possible is provided for and the lever brushes can be used for a very long time. The bearing bush can be connected in a suitable manner to the shaped body during the manufacturing process or by subsequent pressing or gluing.
It is true that there are already constructions in which simple graphite pins are held by a lever arm bent from a sheet metal. These, however, have the drawback that the connection point is arranged on the sheet part. Hence, when the connection between lever arm and graphite pin is established, special care must be taken for ensuring an adequate electrical transition. This, however, will entail costs making the whole construction more expensive. Despite the use of a stable bearing bush, the connection point in the present invention is still held on the shaped body and thus in direct electrical contact with the brush contacting surface. Expensive soldering points, or the like, are not needed. Since the bearing bush has a hollow cylindrical shape most of the time, the bush can be embedded into the shaped body at least in part while the shaped body is being produced. The bearing bush is embedded into the molded body over a circumferential angle of  greater than 180xc2x0, if possible, thereby ensuring a safe anchorage.
Advantageously, the lever brush has a web which is pivotable about the pivot axis and whose face is formed by the brush contacting surface. The height of the web will then substantially determine the wear length of the lever brush, because the brush will normally be used until the web has almost worn down.
The lever brush may comprise a connection nose which is adjacent to the bearing bush and which extends in a direction transverse to the connection line and has the connection point arranged thereon. Since the shaped body can be formed in any desired way, such a connection nose can very easily be mounted thereon. Since the nose is arranged in a direction transverse to the connection line, it can also be adapted to the curvature of the commutator surface so that space can be saved for the connection as well. This means that the connection nose is e.g. bent at the side of the pivot axis which is opposite to the brush contacting surface, i.e. towards the commutator.
Preferably, the connection point is formed by a flexible lead firmly anchored in the shaped body. In particular, a flexible lead which is braided from thin copper wires can very easily be embedded into the shaped body with enough stability. Furthermore, it is sufficiently flexible and establishes a good contact, for example, on contact lugs of the motor connections.
To make sure that a spring element which applies the press force to the lever brush is reliably secured to the lever brush, an undercut which is engaged by the one end of the spring element may be provided in the shaped body. Thanks to the direct engagement of the shaped body, additional components that would make the arrangement more expensive need not be provided for anchoring the spring.
Advantageously, the lever brush may have a circular arcuate back which is concentrically arranged relative to the pivot axis and passes into the undercut, and a roll-type strip spring is guided along in portions on the back and engages with its unrolled end into the undercut and is arranged with its rolled end in a pocket of a housing section. Although it is possible to use other spring elements, such as torsion springs, the roll-type strip spring has turned out to be very well suited, in particular, because of the spring excursion varying with the wear of the brush. In particular in the new state of the lever brush, the unrolled end section of the roll-type strip spring substantially rests in planar fashion on the back of the lever brush, thus hardly occupying more space in the extended position than later in the retracted position.
Since the roll-type strip is guided along the back of the lever brush, the rolled end of the roll-type strip spring can also be arranged in a very space-saving manner and very close to the commutator, which results in a further reduction of the size.